One thing that is very important in hell is fire resistance. The fire traps in Gehennom are particularly nasty because, unlike normal fire traps, they decrease your maximum HP unless you're resistant. You should also fireproof your armor (by reading a scroll of enchant armor while confused for example) and put all items that could burn or boil into a bag, so they won't be damaged or destroyed.
As to the level drainers: The best defense against level drain attacks is magic cancellation. If you have an item which offers level 3 magic cancellation, this will cause drain attacks to fail in most cases, making level drainers much more harmless.
For fighters it's also a good idea to have a silver saber as those are especially effective against demons and the undead (well, most undead - zombies don't care, but zombies aren't really dangerous at this stage anyway).
Also note that you can eat wraith corpses to gain levels.
General things you should do before you enter the end game:
Get your naked AC down to 2 or lower with divine protection (you can get it down to 0 quite easily via donations).
Have magic resistance and reflection.
Be fast (or preferably very fast)
Make sure all of your stats (except charisma) are close to or at their maximum
Enchant most of your armor to +4 or +5.
If you're a fighter, enchant your weapon to +6 or +7.
If you're a caster make sure that all your important spells are recent in your memory or you have the spellbook in your bag of holding. You do not want to forget magic missile while battling The Wizard or Death.
Make sure you have a means to detect portals (not important in Hell, but once you leave the dungeons, that's gonna come in handy and you do not want to have to go searching for gold detection scrolls after you have woken The Wizard).
You should also have a reliable way to cure sickness (blessed unicorn horn or cure sickness spell) as there will be a couple of enemies who can make you terminally ill.
The most important thing to note is that identifying items in one game only helps you in that game. If ruby rings were rings of invisibility last game, but you died and started a new one, you know NOTHING about them. The exceptions are the trivial ones: potions of water and blank scrolls/spellbooks.
The second most important thing about identifying items is that each item can be blessed, uncursed, or cursed. If your role is Priest, then the blessed/uncursed/cursed status is automatically revealed to you for all items. If you are of any other role, then you need to utilize alternative methods to determine the status. The available options are:
- You can figure out which items are which by dropping them on an altar (as long as you are not blind, so that you can see the flashes of light).
- Reading a scroll of identify will also point out the status of the item.
- As mentioned in Durathor's answer, pets will avoid walking on cursed items that are on the ground, only moving "reluctantly" if they ever stand on it. You can use this from the start to avoid an early demise from using cursed equipment.
Blessed items and uncursed items are much safer to use than cursed ones. Don't drink any cursed potions, read cursed scrolls, or put on any cursed rings/armor unless you know exactly what you are doing.
Rings and armour
Conversely, if you have rings or armor that you know is not cursed, you can put it on and take it off right afterwards. For certain types of equipment (ring of invisibility, gauntlets of dexterity, boots of levitation, etc) this may identify it immediately. Others will have clear effects, allowing you to #name them yourself. (Ring of conflict, gloves of fumbling, etc) This will also tell you how heavily enchanted a piece of armor is.
Note that certain types of rare headgear can still cause problems, even if they are not cursed. The helm of opposite alignment will lose you all your divine protection, and will block you from doing the quest. Anything that looks like a "conical hat" should be avoided unless you are a wizard, in which case you may want to wear the expensive type.
Potions
Potions provide you with some interesting options.
- Clear potions are always water. However, they may be blessed (holy water) or cursed (unholy water), which have important uses. You can find out what kind of water you have by dropping the potion on an altar (they won't break, promise).
- You can try #dipping a unicorn horn into a potion. Most bad potions will be neutralized this way. If you dip a unicorn horn into a brown potion, and the brown potion clears/changes color, don't drink any brown potions.
However, just because the potion did not change, does not mean it is safe to drink. Unicorn horns don't detect everything. (sleeping and acid, for example). Also, a potion of polymorph will cause you to lose your unicorn horn as it turns into something else. This can be prevented by dipping any other item into the potion first.
- If you find a food shop, you can try dropping your potions there. Anything that the shopkeeper offers to buy from you is a "food potion." In practice, that means water, fruit juice, or booze.
- Monsters will drink helpful potions (speed, healing, invisibility) and throw harmful potions (paralysis, blindness, acid) at you.
When in doubt, wait for this to happen instead of drinking random potions. The penalties are much less severe, especially if the potion was cursed.
- If a potion was dropped by a nymph, and it's not one she stole from you, chances are that it's object detection.
- The potion of oil is unique in that you can apply it to light it on fire like a lamp. No other potion is a valid target to apply.
Scrolls
For scrolls, I typically end up relying on price. As with all items, more expensive scrolls do 'bigger' things. Some specifics on scrolls:
- First level of Sokoban always has two scrolls of earth in the same spot.
- If you find a scroll in a one-square "closet", it will be a scroll of teleportation.
- Scrolls of identify are really cheap. Cheaper than blank scrolls. This makes them pretty easy to identify.
For more information, including a very detailed price ID guide, see http://www.steelypips.org/nethack/id_faq.html
Best Answer
There are four main things that help in hording items: getting a bag of holding, keeping a stash, travel improvements, and value assessment.
Bag of Holding
These can be purchased in tool shops and other stores, can sometimes be found lying around in the Gnomish Mines, and there's a 50% chance you'll get one at the end of the Sokoban levels. Refer to this map for reaching the Mines and Sokoban, but simply put they're fairly early. The earlier you get one, the easier everything becomes.
The bag of holding is a container, which automatically expands your carrying capacity. Unlike your inventory which is limited by the number of letters in the alphabet, an infinite number of items can be stored in a container. Of course, an infinite item quantity is an infinite weight, which is where the second benefit comes in that a bag of holding reduces the weight of its contents. An uncursed bag of holding reduces the weight of items by 1/2, while a blessed one reduces the weight by a whopping 3/4!
Bags of holding make it a lot easier to both carry all the essentials you need to survive in the dungeon, as well as traffick heavy goods across long distances. Hope to get one as early as possible. However, there are some dangers involved with what you put into a bag of holding: read up on them here.
Keeping a Stash
It's indeed unreasonable to try and carry everything you can. So what is important is to have a place to safely store your stuff. Keeping a stash often is tasked with a few points.
These are the main points of stashing. All it needs to do is hold onto items and keep them safe.
Travel Improvements
Keeping a stash is worthless if you can't use it! You need to have better methods of travel across the levels. This isn't really limited to hording items, but it's something I can cover as a relevant tangent. Just consider 3 commonly accessible methods of easing your travels (I'm not going to cover the Eye of Aethiopica since it's rather specialized and already covered up above).
Value Assessment
The final point to knowing how to effectively horde items is knowing what to horde. Equipment, such as weapons and armour, are often useless to keep. They sell pitifully compared to their weight, and you rarely need to keep any equipment that isn't immediately useful to you. Also try to figure out the usefulness of items by unconventional means. Reduce not just how much you have to carry, but how much you need to stash as well.
And finally, consider carrying no more than one copy of heavy items like potions when unidentified. Identifying one serves to identify them all, and simply rely on one or more stashes to keep the extras.